1. Technical Field
The present invention relates to an optical element having a light entering surface and a light exiting surface substantially parallel to each other.
2. Related Art
Various optical elements have been used in optical pickups, liquid crystal projectors, and other devices. For example, a polarization separating element (a PS conversion element) is given as an example of such optical element. The polarization separating element is formed with a polarization separating film and a reflecting film alternately provided between a plurality of sheets of glass. A portion of light incident on from a light entering surface is transmitted through the polarization separating film and outputted from a light exiting surface after a phase of the light is changed by a phase plate while the light reflected by the polarization separating film is outputted from the light exiting surface after reflected by a reflecting film.
The polarization separating element commonly has the following structure: the plurality of sheets of glass includes a first glass, a second glass, and a third glass; each glass has oblique surfaces formed along a direction obliquely intersecting with the light entering surface and the light exiting surface; the polarization separating film is formed on the oblique surface of the first glass; the polarization separating film is bonded to one oblique surface of the second glass with a bonding layer interposed therebetween; the reflecting film is formed on the other oblique surface of the second glass; and the third glass is bonded to the reflecting film with the bonding layer interposed therebetween. JP-A-2000-298212 is a first example of related art while Japanese Patent No. 4,080,265 is a second example of related art. The first example discloses the polarization separating element in which the phase plate is bonded to the light exiting surface of the glass with the bonding layer interposed therebetween. Further, the second example discloses the polarization separating element in which the phase plate made of biaxially oriented plastic film is bonded to the polarization separating film with the bonding layer interposed therebetween.
In the first example, two types of glass are prepared and bonded by considering a thickness of the bonding layer, which is several tens of micrometers, so that intervals between the polarization separating films and the reflecting films adjacent to each other are equal. In the second example, two types of glass are prepared and bonded by considering a thickness of the phase plate so that intervals between the polarization separating films and the reflecting films adjacent to each other are equal.
In the first and second examples, it is required to prepare two types of glass having different plate thicknesses between the oblique surfaces facing each other. This causes an increase in manufacturing complexity. In other words, it is required to prepare two types of glasses having different reference values (the process target values). This results in lowering manufacturing efficiency. Particularly, since a thickness of the bonding layer is several tens of micrometers, it is hard to accurately control the thickness of the bonding layer to manufacture the polarization separating element. Further, the biaxially oriented plastic film is used as the phase plate in the second example. It lowers film thickness precision of the plastic film. As a result, accuracy of the polarization separating film is also lowered.